Behavior of multiple barrier insulation systems under impulse conditions

Deals with prebreakdown and breakdown behavior of composite liquid-solid-insulation with multiple solid-barriers in liquid gaps submitted to impulse voltages. Experiments have been carried out in strong and weakly inhomogeneous fields with gap distances from 8 to 34 mm, respectively, using impulse voltages up to 700 kV. Configuration was close to transformer insulation with up to three pressboard layers of 2 mm thickness each in the liquid gap. Comparisons between streamer behavior in single and multiple barrier insulation are presented. Streamer inception and propagation was visualized via highspeed shadowgraph photography. Compared to pure liquid insulation an increase of the insulation ability of 470% of the pure liquid gap was observed, most likely due to field homogenisation through solid barriers. Streamer inception field strengths differ little from phenomena in pure oil gaps. Microdischarges appear at board surfaces closest to electrodes, leading to Lichtenberg-type figures on the board surface which cause degradations of the board. Investigations were made with pre-stressed and new pressboard.